I can only speak for the credit union I work with, but their app is largely better than any web based banking site I have used. From what I can find it is considered very well built, and secure, not just smooth to work with, simple UI, and have never given me single issues using it as designed.

I find myself on their site only for reading things. Like loan policies, investment stuff, etc. cause those types of documents just work better for me on a desktop.